This invention relates to a method and an apparatus for continuous, gravimetric metering and mass flow determination of flowable materials, with a metering device, especially a metering rotor with determination of the instantaneous mass flow.
Such a system for continuous gravimetric supply and/or metering of bulk goods is known from DE 4 023 948 A1, wherein a metering rotor balance according to DE 3 217 406 A1 or EP A 0 198 956 is utilised. The metering device following a bulk goods feeder is arranged in a closed pneumatic conveyor path and a collector device with a cell wheel lock supported on load cells is connected afterwards. A computer-controlled, central metering system is used for the appropriate regulation of the desired feed amount per unit time (feed strength), such as is described in DE 3 217 406 A1, wherein the weighing signal of the hopper weighing cells serves as an input signal and the speed of the metering rotor and optionally the cell wheel lock for the bulk goods feed are regulated. However there is some regulation dead time until adjustment up to the set-point feed strength.
This regulating system is described in more detail in the above-cited EP A 0 198 956, wherein the bulk goods mass acting instantaneously in the rotor weighing path in the metering rotor balance is detected in the metering rotor balance, from which the bulk goods mass throughput is obtained by multiplication with the angular velocity of the metering rotor. The weighing electronics delay passing on the current weight value of the bulk goods mass (loading) present at the moment on the rotor weighing path (measuring path) up to a particular transfer point, so that the rotor angular velocity can be varied shortly before the discharge of the bulk material to the pneumatic conveyor line in accordance with the predetermined set-point conveyor strength, i.e. the metering rotor is accelerated or delayed. A relatively high metering accuracy results, which has generally proved itself in metering powdered bulk goods, for example in coal dust metering in rotary cement kilns or for metering additives in flue gas purification.
With certain bulk goods however, such as additives for admixture in flue gas purification from power plants, it has to be recognised that these bulk goods can tend to xe2x80x9cshootingxe2x80x9d or formation of bridges, depending on the degree of moisture, fluidisation, fineness and the like, so that random or strongly pulsating bulk flow fluctuations (loading differences) can occur in the bulk goods feed. In this case the previously described metering device can hardly compensate for the short-term mass flow fluctuations on account of the inertia, since the metering rotor has a substantial own mass of some hundreds of kilograms, especially in a design resistant to pressure surges, so that the required changes in angular velocity for keeping the feed strength constant with strongly fluctuating mass flows, i.e. strong acceleration or deceleration of the metering rotor, are hardly possible on account of the mass inertia.
These plus/minus fluctuations can be partially compensated in the first-mentioned system for feeding and/or metering bulk goods, in the way of forming an average value, in that a collecting container supported on load cells is provided. However, such a compensating container increases the expense of construction and the demand on space.
In consequence the invention is based on the object of providing a method and an apparatus for continuous gravimetric metering and mass flow determination, especially of bulk goods, with which an improved metering accuracy is attained in a simple way, even with strongly fluctuating conveyer material feed.
One aspect of the invention involves an apparatus for continuous, gravimetric metering and mass flow determination of flowable materials. A metering device is configured to determine instantaneous mass flow at an anticipatory control point ahead of a delivery point at which flowable material is discharged. The discharge of the flowable material is controllable by altering a speed of rotation of the metering device. A metering controller is connected to the metering device and configured to detect an actual speed of rotation. Discharge at the delivery point is regulated in dependence on mass flow deviations at the metering device by variable location of the anticipatory control point.
Another aspect of the invention involves a method for continuous, gravimetric metering and mass flow determination of flowable materials. A metering device determines an instantaneous mass flow at an anticipatory control point ahead of a delivery point for the flowable material. The discharge of the flowable material is affected by altering a speed of rotation of the metering device. A distance of the anticipatory control point ahead of the delivery point is determined to match at least one of a current actual speed and a loading of the metering device.
By regulating the metering device and thus the discharge for regulating the feed strength at an anticipatory control point calculated from the current actual speed and in direct dependence on the instantaneous mass flow determined at the metering device, a highly accurate compensation for disturbing values results still before transfer of the bulk goods at the point of delivery or process system, so that a particularly high short-term metering accuracy is achieved. Regulation of the actual feed strength to the set-point value as adjusted is thus effected with practically no dead time, since it is determined accurately when or at what angle the mass deviation will arrive at the delivery point and this can be compensated for accordingly by increasing or reducing the conveyor speed.
This is particularly important in coal dust metering for achieving a constant burning process or in flue gas purification. This compensation for disturbing values in the metering ahead of transfer of the conveyed material into the chemical or thermal process thus facilitates a particularly high short-term metering accuracy in the manner of anticipatory regulation, wherein the use of a metering rotor in particular allows a highly accurate mass flow determination through a wide range of adjustment of the set-point feed strength.
Preferred embodiments are the subject matter of the dependent claims, wherein in particular the possibility of monitoring by measurement of the actually attained actual speed at a monitoring point and a following correction is especially advantageous, since residual regulation deviations can by avoided by this.